1. Field of the Invention
The present invention relates to an optical system of reflected light path for a magneto-optical head, and more particularly to an optical system for a magneto-optical head comprising very few parts that can be manufactured and assembled at low cost.
2. Description of Prior Art
With advancements in Hi-Tech and diversification in the media for information or data storage and retrieval, magneto-optical disks are becoming more popular in applications for data storage and retrieval. Most of the optical disks currently used for information or data storage and retrieval are classified into a group we call WORM, meaning "write once, read memory".
Basically, it is necessary for the optical system of a magneto-optical head to detect three types of signal. The positions of an objective lens and an optical disk are detected from the focus signal. The precision-positioning of the control tracks of the objective-lens actuator is made possible by the detection of the track signal. When both the focus and track signal are accurate, the RF signal of an optical disk can be readily detected. To perform all these functions, conventional magneto-optical heads have a very complicated design and require a large number of optical parts to make up their optical systems. Due to the complex structure and the large number of optical parts employed, conventional magneto-optical heads require a lengthy process for assembly and incur a high cost of manufacture.
TABLE 1 ______________________________________ Magneto- Ease of Units of BS Units of Other Optical heads Assembly or PBS lens Components ______________________________________ Con 1 difficult 3 3 knife-edge Con 2 difficult 2 3 knife-edge Con 3 difficult 1 3 wedge prism Con 4 difficult 1 2 wedge prism Con 5 difficult 3 2 knife edge PI 1 easy 1 1 PI 1 BS PI 2 easy 1 1 PI 2 BS ______________________________________ Con: Conventional PI: Present Invention PBS: Polarized beam splitter BS: Beam splitter
Referring to Table 1, a table showing different characteristics of reflected light paths for different magneto-optical heads, different designs of reflected light paths for conventional magneto-optical heads (referring FIG. 1 through FIG. 5) and the magneto-optical head with the present invention (referring FIGS. 7 and 8) are compared and contrasted. The table clearly shows the simplicity in structure and ease in assembly for the magneto-optical head with the present invention in comparison to conventional magneto-optical heads. Most of the conventional magneto-optical heads employ more beam splitters than the magneto-optical head with the present invention. The magneto-optical head with the present invention employs only one lens while the conventional magneto-optical heads employ more than one lens (some even employ as many as three lens).
FIG. 1 through FIG. 5 are drawings showing different conventional magneto-optical heads. From these diagrams, it can be found that the designs of the optical systems for the conventional magneto-optical heads are complicated and require numerous optical parts. Therefore, the time of assembly for these conventional magneto-optical heads is longer and their cost of manufacture is higher.
FIG. 1 is a diagram showing a first conventional magneto-optical head. The first conventional magneto-optical head is shown to comprise a laser diode 20, a collimator 21, a prism 22, two beam splitters 23, 24, an objective lens actuator 25, a compensation plate 27, a half-wave plate 28, two polarized beam splitters 29, two signal detectors 30, 31, a differential amplifier 32, a knife-edge 33, a focus error detector 34, and a tracking error detector 35. The arrows shown in the diagram indicate the paths of the laser beam. Numeral 26 denotes a magneto-optical disk.
FIG. 2 is a diagram showing a second conventional magneto-optical head. The second conventional magneto-optical head is shown to comprise a quadrant photo diode 40, a quadrant analyzer 41, an electromagnet 42, an objective lens actuator 44, a laser diode 45, a collimator 46, a prism 47, a beam splitter 48, and a half-wave plate 49 and two PIN photo diodes 50. The dotted lines shown in the diagram indicate the paths of the laser beam. Numeral 43 denotes a magneto-optical disk.
FIG. 3 is a diagram showing a third conventional magneto-optical head. The third conventional magneto-optical head is shown to comprise a polarized beam splitter 50, a half-wave plate 51, a beam splitter 52, a mirror 53, an objective lens 54, a first and second collimating lens 55a and 55b, a laser diode 56, a PIN photodetector 57 for signal and focus error detection, a wedge prism 58, and a PIN photodetector 59 for signal and radial error detections. The arrows shown in the diagram show the paths of the laser beam.
FIG. 4 is a diagram showing a fourth conventional magneto-optical head. The fourth conventional magneto-optical head is shown to comprise an objective lens 72, a deflection prism 73, a polarized beam splitter 74, a split photodetector 75, a wedge prism 76, a quadrant photodetector 80, a half-wave plate 81, a laser diode 82, a collimating lens 83, an APC photodetector 84, and a beam splitter 85. Numerals 70, 71 denotes an electromagnetic coil, and a magneto-optical disk.
FIG. 5 is a diagram showing a fifth conventional magneto-optical head. The fifth conventional magneto-optical head is shown to comprise a laser diode 91, a collimator 92, three polarized beam splitters 93, 94, 95, a convex lens 97, a cylindrical lens 98, two quadrant PIN photodiode 99 and 111, a half-wave plate 110, and a PIN photodiode 112. The dotted lines show in the diagram show the paths of the laser beam.